1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a perforating system having a system for compensating pressure inside a perforating gun body with wellbore pressure.
2. Description of Prior Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. In FIG. 1 an example of a perforating system 4 is shown. For the sake of clarity, the perforating system 4 depicted comprises a single perforating gun 6 instead of the typical multitude of guns. The perforating gun 6 is shown disposed within a wellbore 1 on a wireline 5. The perforating system 4 as shown also includes a service truck 7 on the surface 9, where in addition to providing a raising and lowering means, the wireline 5 also provides communication and control connectivity between the truck 7 and the perforating gun 6. The wireline 5 is threaded through pulleys 3 supported above the wellbore 1. As is known, derricks, slips and other similar systems may be used in lieu of a surface truck for inserting and retrieving the perforating system into and from a wellbore. Moreover, perforating systems may also be disposed into a wellbore via tubing, drill pipe, slick line, coiled tubing, to mention a few.
Included with the perforating gun 6 are shaped charges 8 that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge 8 at very high velocity in a pattern called a “jet” 12. The jet 12 perforates the casing and the cement and creates a perforation 10 that extends into the surrounding formation 2.
FIG. 2 illustrates in side partial sectional view an example of a prior art perforating gun 6. The perforating gun 6 includes an annular gun tube 16 in which the shaped charges 8 are arranged in a phased pattern. The gun tube 16 is coaxially disposed within an annular gun body 14. On an end of the perforating gun 6 is an end cap 20 shown threadingly attached to the gun body 14. On the end of the perforating gun 6 opposite the end cap 20 is a lower sub 22, also threadingly attached to the gun body 14. The lower sub 22 includes a chamber shown having an electrical cord 24 attached to a detonator 26. A detonating cord 28 is included shown having an end connected to the detonator 26 and wound around the gun tube 16 for connection to the lower end of each shaped charge 8. As is known, an associated firing head (not shown) can emit an electrical signal that transferred through the electrical cord 24 and to the detonator 26 for igniting the detonating cord 28 to then detonate the shaped charge 8.
An annulus 18 is formed between the gun body 14 and gun tube 16 that typically is at a pressure substantially the atmospheric pressure of the location where the perforating gun 6 is assembled—which is generally about 0 pounds per square inch gauge (psig). Thus at surface 9, no differential pressure is exerted on the gun body 14. However, wellbore fluids in a wellbore 1 can generate static head pressure that often exceeds 5,000 psig. Thus when the perforating gun 6 is deployed at depth within the wellbore 1, the gun body 14 will experience a significant differential pressure. The large pressure difference across the gun body 14 wall requires thicker and stronger walls to enhance their strength, as well as robust seals in a perforating gun 6.